The objective of this research is to determine the cellular requirements for activation of antibody-producing (B) cells by thymus (T) dependent (TD) and T independent (TI) antigens by determining 1) the cellular requirements for induction of secondary IgG responses to Type III pneumococcal polysaccharide (S3) in nude mice, 2) the antigen requirements for priming of B and T cells for secondary IgG responses to S3, 3) the cellular requirements for induction of IgG-producing memory B cells for other TI and TD antigens, 4) the cellular requirements for elicitation of Ig G memory responses by TI antigens and 5) the antigen requirements for priming and elicitation of IgG memory responses. Antibody responses will be measured by the Jerne plaque assay and by hemagglutination. Effects of T cells on B cell responses will be measured by transferring column-purified T cells from thymus-bearing littermate mice to athymic (nude) mice or by transferring T cells with B cells in adoptive transfer experiments in irradiated mice.